cluster-merge: Add the --skip-stop-instances opt
cluster-merge: Add the --skip-stop-instances opt

This option allows to do a check for running instances on the mergee
clusters instead of stopping them.

Signed-off-by: Andrea Spadaccini <92d490849ac1b40cd8e8878e4d94603bebdb089f@google.com>
Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

onponomarev/ganeti,andir/ganeti,grnet/snf-ganeti,ganeti/ganeti,mbakke/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,ganeti/ganeti,yiannist/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,yiannist/ganeti,andir/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,mbakke/ganeti,apyrgio/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,leshchevds/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,dimara/ganeti,andir/ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti